Negative resistance coupling device



July 18, 1950 J. c.o'BR1EN NEGATIVE RESISTANCE couPLING nEvIcE Filed Sept. 15, 1945 z/ ATTORNEY NVENTQR. BY d /uuo/ wZOILONU-Z /m 521ml, fw ww :l @GOA QZSDMQ. loog GZELHEwZE. \S

Patented July 18, 1950 "UNIT-Eo STATES PATENT OFFICE NEGATIVE RESISTANCE COUPLING DEVICE John C. OBrien, Rochester, N. Y., assignor to General Railway Signal Company, Rochester,

Application September 14.3, 1945, Serial No. 616,055

This invention relates to communication systems adapted for use -by mobile units such as tra-ins traveling along a railroad, or` the like, wherein communication is established-by inductive coupling between vthe mobile units and a transmission line' .or other conductive medium` adiacent the right-of-way; and more particularly 2 Claims.. (Cl. 179-25) pertains to the provisionof Amore effective inductive coupling in'such systems.

It has been heretofore proposed to provide mobile units, such as train-s, with 'communication apparatus of the type wherein the transmitting. station modulates a y.carrier frequency signal: i

which is supplied toa loop radiator -or coupler to produce an electromagnetic `field which acts to vtransfer signal energy to the line wires or other conductors extending .more or less parallel tothe right-oiv-way; and similarly, wherein the f In providing systems Qithis character, it will be ,read-ily apparent that the degree of coupling between the loop couplers or antennas .and the transmission line or other conductors `along the right-oi-way is very-.small and for this `reason the organization should 'be made as -eiicient as possible. In accordance with the present invention, it is proposed to improve the transmitting and receiving organizations in such a way as to materially improve the. inductive coupling charl acteristics and effect the ,maximum transfer of signal energy for .the particular .level of energy.

employed by the transmitter. c.

In a system of the type above described, the resistive characteristics of thetransrnission line and also the metal ,parts of the associated vehi.

cle are eiTect inserted as positive resistance l into the inductively coupled circuits of the transv mitter and receiver units. `Under these.condi-v tions, there is usually. an excessive value of posi-.r

tive resistance coupled into the circuits, therebytending to reduce the `eilciency-ot energy transfer both in transmission and reception. Itis thus proposed in accordance with the present invention to cancel thisexcessive value oify thecoupled positive resistance byintroducing a confA trolled value of negative resistance.

More particularlygt Vis proposed that the can-l collation of positive resistance reiiected into the. inductive coupling unit may be obtained by the provision of the proper quantity of negative re: sistance supplied by a suitable generator of the negative resistance type.

Thus, it may be said that one of the objects of the present invention is to so increase the efiiciency of .energy transfer in systems of the character described that communication may be more effective and carried on over greater distances than heretofore considered feasible.

Other purposes and characteristic features of the present invention will be in part obvious, and in part pointed out, as the description of the invention progresses.

In describing the invention in detail, reference will be made to the accompanying drawing illustrating a transmitter unit and a receiver unit both constructed in accordance with the present invention and capable of providing improved transmission of intelligence between their respective stations by the inductive transfer of energy via an adjacent transmission line or other con duotive medium.

For the purpose of simplifying the illustration and facilitating in the explanation, the various' parts .and circuits constituting the embodiment of the invention have been shown diagrammati' such as electron tubes, microphone, batteries,

resistors, condenserS, :and the like, have been illustrated in a conventional manner using symbols rather than illustrating 'these ydevices in detail.

With reference to` the drawing, a vtransmission line lor other conductive medi-um is indicated at I as extending overa considerable distance.V

At the left hand side vof the draw-ing, a rmobile transmitter unfit is illustrated -as .being induce. tively associated with the transmission line or other medium l while on -the rig-ht hand `side of the 'drawing a mobile receiver -unit has been show-n as inductively associated Awith the vtrans-.-

mission line.

The mobile transmitter unit comprises 'in goneral `an .audio amplifier, -a carrier frequency ,oscil.-

lator, va modulator adapted to `:modulate the car.- l

rier frequency in accordance with the principles ,of frequency modulation .by the .audio .messages spoken into the conventionally illustrated microphone. The output of the modulator and any additional ampliiier is supplied through suitable connections and condenser C to the control grid of a power amplifier designated PA in the drawings. rIhe grid leak resistor RI is used to provide the bias and thus provide the amplitude of the input to the power amplier PA. This power amplifier PA, which supplies the primary Winding 2 oi the transmitter coupler TC with the frequency modulated carrier signals, is illustrated as being a tetrode tube having a heater type cathode, a control grid, a screen grid and a plate. condenser CI to the cathode in the usual way, and a variable condenser C2 is used for tuning the output winding 2. Plate potential is supplied by a suitable plate source designated PSI and connected through a radio frequency choke RFI. A by-Dass condenser C3 is connected between the upper terminal of the choke RFI to ground.

The transmitter coupler TC has windings 3 and Il which act as the inductances of the tuned plate and grid circuits of an oscillator or regenerative amplifier OC. A variable condenser C is connected across these inductances and a condenser C connects one terminal of the tuned tank circuit to the grid of oscillator tube OC while the grid is connected through a resistor R2 to ground. The value of this resistor R2 determines the amount of bias normally on the grid of the tube OC and thus limits the amplitude of oscillation in accordance with its value. Resistor R4 provides adjustable control of the total negative resistance available in the output.

The plate potential is supplied to the tube OC by a suitable plate source designated PS2 and connected through a radio frequency choke RFZ to a point between the windings 3 and 4. A bypass condenser C6 is also connected from the midpoint between windings 3 and 4 to ground to complete the circuit to the cathode for the tuned plate and grid circuits.

An output or secondary winding 5 of the transmitting coupler TC is connected to a transmitting loop 6, which in practice will have a plurality'of turns although only one has been indicated in the drawing. Also, this transmitting loop 6 is relatively large so as to produce as much inductive coupling with the wayside conductors as possible in spite of the substantial intervening space.

The mobile receiver unit includes a receiving loop II which is similar to the transmitting loop 6 since it includes a plurality of turns and is of substantial size so as to produce as much inductive coupling as is possible considering the space between the receiving loop II and the wayside conductors represented at I. The receiving loop II is connected in series with an input winding I2 on the receiving coupler RC.

`A regenerative amplifier including a Dentode electron tube RA is inductively coupled to the receiving coupler RC by windings I3 and M having a common connecting tap forming a, connection to the cathode of the amplifier tube RA. The upper terminal of winding I3 is connected to the control grid of tube RA, while the lower terminal of winding I4 is connected through condenser C'I and variable resistor R3 in multiple to ground.

A variable condenser C8 is connected across the windings I3 and I 4 of the receiving coupler RCv to produce a resonated or tuned circuit for the regenerative amplifier RA which is of course The screen grid in connected through 4 tuned for the carrier frequency signals to be received over the line' conductors. The screen grid of the regenerative amplier RA is connected to ground through the condenser CID and to the positive high voltage, through resistor R5, while the suppressor grid is connected to the cathode in the usual way. Plate potential is provided from a suitable plate supply PS3, but is bypassed for the carrier frequency by a suitable condenser C9.

An output winding I5 of the receiver coupler RC is connected to supply the received carrier signals as modified by the regenerative amplier RA to a, conventional carrier frequency amplilocated within the particular vehicles with which they are associated, and for this reason, the transmitting and receiving loops will have to be connected by suitable lead-in wires to their respective transmitter and receiver units.

in free spaces with respect to the trains or other mobile units carrying them. understood that the various devices associated with the transmitter and receiver units are intended to be of conventional character, and the A disclosure has been considerably abbreviated in order to more clearly illustrate the organization forming the present invention.

It is believed that the characteristic features y of the present invention will best be understood by further description being given from the; standpoint of operations and functioning of the various units.

Referring to the drawing, it will be noted that the power amplifier PA in effect acts as a generator or source of power which is inductively coupled through the primary winding 2 of the transmitting coupler TC to the secondary winding 5 supplying energy to the transmitting loop 6. .The loop 6 with its plurality of turns forming the loop is inductively coupled through elecl ouency modulation, is inductively `supplied to the transmitting medium along the roadway. The signals thus produced in the transmission line travel for considerable distances due to the relatively good conducting .characteristics of such line or lines.

These modulated carrier signals, throughout the territory or range of transmission, may b picked up bv one or more mobile receiver units by reason of the inductive coupling oi the receiving loop of each receiver unit with the transmission line. The energy thus received by a mobile unit is supplied inductively to the innut winding of the carrier frequency amplifier. demodulator and audio amplifier apparatus of the receiver.

In the above brief description of the opera.- tion of the communicating organization. the

functioning of the regenerative amplier or oscillator OC and the regenerative amplier RA has been omitted. Thisl has been done for two The mountings of the loops 6 and II are preferably i It should also be reasons, namely. .because their action is a .modifying Vaction on the transmissionand reception and also because it is convenient vto ydiscuss the general principles of the basic functioning of the system before considering such modifying `action in detail.

More specically, it will be Well understood by those skilled in the art, that the electrical characteristics of the transmission line I, the transmitting loop -Ii Vand the .output winding -5 will be reected into the .output circuit of the `power amplifier PA, the lsame as if an equivalent impedance were .inserted vserially -in the plate .circuit of.` the power amplifier PA. In order for the power amplifier P A to effect the greatest transfer. of energy `to its load circuit, the sum of they impedance of its output circuit including the relected vimpedances from the secondary circuits, and the impedances of the primary winding 2, mus-t be matched with the impedance of the arnplier PA. Although this matching of impedances in the output circuit of the power amplifier `-may effect the maximum current now in primary Winding 2, itis also essential that the resistance characteristics of the various circuits involved shall have the proper values in order to obtain the maximum transfer of energy to the Wayside line I. This `is because lof the conditions involved .in .establishing the conditions of critical coupling for the maximum transfer :of energy from .a primary winding to a secondary winding in a resonated circuit.

For a com-plete discussion of critical coupling reference may be `made to Termans Radio Engineersl-Iandbook, pages 155 to 156. However, for .the purposes of the present description, it is sufficient to understand that the maximum transfer of energy .between the primary and secondary windings of a transformer at resonant frequency is dependent both upon the actual mutual coupling between the windings and the matching of .the primary and secondary resistances. This .is expressed .by the .equation ('2wfM)2 Rs *Rp Since are. fn. M QlU-ZvrR113 Qs-ZWRS and K-LpLS Therefore, `critical coupling is expressed by th equation Where M=mutua1 inductance #frequency Rp--primary resistance Rszsecondary resistance Lpprimary induotance lia-:secondary inductance K=.co.efficient of coupling tiueresistance were added in the circuit of w-i-ndv ing .5. .Si-nce 'there Ais no controlfoyer the .re-

6i sistanoe .of the transmission line and adjacent metal bodies coupled into the loop circuit, it is desirable to provide means to compensate for the positive resistance to that extent -necessary to cause the positive resistance of such circuit vto be equal-'to the positive resistance of the primary circuit dependen-t upon the mutual coupling to thereby cause the maximum transfer of signalv energy.

It will also be observed that the inductive coupling between the transmitting loop 6 and the transmission line `I is very loose, so that the coeliicient of coupling will Abe very small. The particular degree of coupling cannot be changed at will, por is it continuously the same, .sincethe mobile unit will be moving along a roadwayadjacent to :the transmission line at varying distaboes dependent upon the contour of the territory and the various circumstances involvedin the construction ofthe transmission line and the roadway.

Thus, it will :be apparent that the resistance characteristics of .the transmission line and the surrounding metal portions of the vehicle carrying the transmitter unit are quite out of control insofar as the selection of their values are concerned. On the other hand, the maximum transfer of energy cannot be effec-ted between the primary Winding 2 and the transmission line l unless the conditions of critical .coupling are established between the primary winding 2 and the :secondary winding `circuit with the va rious 'resi-stances of the transmission line and the surrounding metal `obfects reflected therein. Since the values 4of the reflected resistances .are not controllable by ordinary means, it is 'proposed in accordance with the present invention to render such coupled in or reflected resistances controllable by the provision of suitable values of Vnegative `resistance coupled into the output circuit to compensate for the undesired `portions of the positive resistance in such circuits .in such a way as to provide therequired values of resistance necessary lto give substantially the conditions required for critical coupling regardless of the exi-sting coupling.

To accomplish this purpose, it is `proposed to provide the coils 3 and Il yconnected -to the oscillator OC with `a tuning variable condenser C5. Suitable feed back energy is supplied to the grid through-.condenser C4 so that the oscillator OC is regenerative in character with the effect of vnegatiye resistance appearing Vacross -the outer terminals rof the l.coil-s k3 and il. This negative resistance, which 4can be governed in its value in accorda-nce with .the value of the variable series tank circuit resistor R4, acts to limit the amplitude of oscillations or regeneration of the ampliier IOCI, to'inser-t the necessary yalue of negative resistance to cancel the proper amount of positive resistance present due `to the above explainedcauses. This effective negative resistance is coupled `into ythe coils -2 and 5 through the associated coi-ls 3 and# and is effective to cause the organization to substantially approach the conditions of critical coupling between the foutpu-t vcoil Af2 fand the transmission line I.

The signal energy which is thus inductivelsr produced in the transmission line I travels throughout a considerable distance .causing such transmission yline -to in turn produce lines of magnetic flux within a limited area -of the line throughout such distance.

These .electro mag netio lines act upon the receiving lloop II and `produce a current inthe Winding I2 of the re,- L

ceiving coupler RC, which in turn induces potentials in the winding I3 of the coupler RC. This grid-cathode circuit is resonated by a variable condenser C3 to the carrier signal frequency and the energy therein is supplied to the control grid of the amplifying pentode tube RA. The plate circuit of this tube RA is connected through the winding III, condenser C'I in multiple with variable resistor R3 and condenser C9 to provide an inductively coupled feed back to the grid circuit including the winding I3. This provides a regenerative amplifier so that thev carrier signals received by winding I2 not only directly produce corresponding signals in the Winding I but'also produce a regenerative amplication in windings I3 and I4 which also acts to produce carrier signals in winding I5 of increased amplitude. rThe winding I5 is connected to the usual carrier frequency amplifier, demodulator and audio amplifier, so that its energy produced in accordance with the characteristicsof the picked up energy cause the message to be reproduced by the loud speaker.

This regenerative amplifier RA has its regeneration oontrolled by the variable resistor R3 so that it will not reach the point of self oscillation but does act to in effect insert negative resistance in thecoupling organization so as to cause the conditions of critical coupling to substantially exist between the transmission line I and the receiver input winding I5. In other words, the same principles of energy transfer between the transmission line I and the receiver are present in the coupling organization for the receiver unit as explained in connection with the transmitter unit. That is, in order to obtain the maximum transfer of energy between the transmission line I and the input winding I5, it is necessary to approach the conditions of critical coupling, and since due to the presence of metal structures and the inherent resista-nce of the transmission line, there is usually too much positive resistance coupled into the circuit' so as to obtain the conditions of critical coupling. Thus, in accordance with the present invention the insertion of the regenerative amplilier RA acts to produce an effective negative resistance for cancelling the undesired portion of theH positive resistance coupled into the circuit and thus substantially cause an improvement in the coupling organization.

The main difference between the receiver unit and the transmitter unit, is the fact that in the transmitter unit the element producing the negative resistance may be of the oscillatory type, which in its oscillations locks into and follows the carrier frequency as produced through the power amplifier; whereas in the receiver unit, since the received energy is of such small degree, it is impossible to provide a regenerative oscillator which will lock-in with. the carrierfrequency of the received signal so that the amplier is of the controlled regenerative type which is maintained at a condition of operation optimum for the degree of coupling encountered. Thus, the negative resistance characteristics of a regenerative or oscillatory circuit are used in both the transmitter unit and receiver unit to compensate for an eX- cessive amount of positive resistance coupled into the circuit in such a way as to improve the coupling characteristics as between the respective units and the transmission line carrying the transmitted signals.

In the drawings, the oscillator or regenerative amplifier OC has been shown as being of the well known Hartley type of oscillator. Also other conventional circuit arrangements have been shown.'

However, it should be understood that the different circuit organizations may be changed "and other well known circuit combinations employed to accomplish the same results, functions and purposes as disclosed herein without in any way departing from the invention. The purpose of the present invention is to be able to compensate for undesired positive resistance, which is especially troublesome in connection with transmitter and receiver units employed on railway cars and' the like where large sections of metal are present adjacent the loop couplers, and by so doing, to improve the effectiveness of coupling to the adjacent transmission line. For this reason it is obvious that the present invention is especially adapted for railroad use where a transmitter or receiver unit may be located upon a locomotive or a 4steel practice, without in any manner departing fromA the spirit or scope of the present invention except as limited by the present claims.

What I claim is:

l. In a communication system of the carrier frequency type wherein a mobile transmitter is inductively coupled to a wayside conductor for the transmission of carrier signals over the conductor with such inductive coupling resulting in the reection of the positive resistance of said conductor into said transmitter organization, said transmitter including a carrier frequency oscillator, means for frequency modulating the carrier frequency to constitute carrier signals, a power amplifier for amplifying said carrier signals and having an output circuit, a loop, a transformer having primary and secondary windings, said primary winding being connected to said output circuit of said' power amplifier and said secondary winding being connected to said loop, a negative resistance device comprising a locking-in oscillator having an output circuit and a feed-back circuit, another winding on said transformer connected to the output circuit of said locking-in oscillator for inductively coupling its negative resistance effect into said output circuit of said power amplifier,

said locking-in oscillator thereby locking into synchronism with saidl frequency modulated carrier signals, and manually adjustable means connected in the feed-back circuit of said locking-in oscillator for selecting its negative resistance output required to compensate for the positive resistance reflected into said output circuit of said.

power amplifier to that degree necessary to cause the maximum transfer of energy from the output circuit of said power lamplifier to said wayside conductor.

2. In a carrier frequency type communication system having a mobile transmitter mounted on a movable vehicle constructed of metal parts, said transmitter comprising in combination, a loop inductively coupled to a line circuit, an oscillator, a modulator, and a power amplifier having its output circuit inductively coupled to said loop through a transformer coupling mean-s for effecting the transfer of modulated signals to the line circuit, said transformer coupling means at the same time coupling positive resistance from said line circuit an-d other objects into the output circuit of said power amplifier, a negative resistance device, another winding on said transformer coupling means connected to said negative resistance device for inductively coupling its negative resistance effect into the output circuit of said power amplier to in part compensate for the positive resistance reiected into its output circuit through said transformer coupling means from said line circuit and the other objects adjacent said loop, and adjusting means connected to said negative resistance device and adjusted to the proper degree to cause the necessary output of said negative resistance device to compensate for that amount of positive resistance inductively coupled from said line circuit and other objects to cause the resultant resistance coupled into the output circuit of said amplifier to be substan- 10 tially equal to the positive resistance of the output circuit of said power amplier to thereby produce the conditions of coupling for effecting a maximum transfer of energy between said power amplier and the line circuit.

JOHN C. OBRIEN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,387,986 Hull Aug. 16, 1921 1,687,253 Latour Oct. 9, 1928 1,963,751 Llewelly June 19, 1934 2,121,877 Linsell June 28, 1938 2,131,388 Place Sept. 27, 1938 2,235,565 Roberts Mar, 18, 1941 2,398,741 Halstead Apr. 16, 1946 

